void insertion_sort(int arr[], int len) {
    int i, j, key;
    for (i = 1;i < len;i++) {
        key = arr[i];
        j = i - 1;
        while ((j >= 0) && (arr[j] > key)) {
            arr[j + 1] = arr[j];
            j--;
        }
        arr[j + 1] = key;
    }
}